1. Field of the Invention
The present invention relates generally to a wireless charging apparatus and more particularly, to a method for controlling charging power and a wireless charging apparatus for the same.
2. Description of the Related Art
Mobile terminals should be continuously provided with power, in order to operate. To this end, mobile terminals are equipped with a rechargeable battery, which is generally recharged by a charger. To support portability of the mobile terminals, studies have been conducted on non-contact or wireless charging. For wireless charging, the position where a mobile terminal, or a target to be charged, is placed on a charging deck of a wireless charging apparatus for charging the mobile terminal is a key factor that determines charging efficiency.
In the wireless charging system, the impedance varies depending on the position of the mobile terminal on the wireless charging apparatus, as described in detail with reference to FIGS. 1, 2A and 2B. FIGS. 1, 2A and 2B illustrate changes in frequency that occur when a target 110 to be charged is placed at a position “A” and at a position “B” of another target 120 on a charging deck of a wireless charging apparatus 100.
Referring to FIG. 1, the impedance will vary depending on the position where the target to be charged is placed, and the number of targets to be charged. Based on a reference frequency of 6.78 MHz, an effect of frequency changes on a target 110 placed at position A are shown in FIG. 2A, and frequency changes on the other target 120 placed at the position B are shown in FIG. 2B. Although the wireless charging apparatus 100 should output 6.78 MHz as a reference frequency, a frequency capable of providing improved efficiency for each different position will change, since the frequency varies depending on the position of the target to be charged.
Experimental results indicate that different positions have different efficiencies, as shown in Table 1 below.
TABLE 1ReferenceInputOutputEfficiencyFrequencyVoltageCurrentPowerPower(Rx output/Position(MHz)(V)(mA)(mW)(mW)Tx input)A6.7872801960108655.4%B6.787204142872951.1%
Referring to Table 1, an efficiency difference between targets 110 and 120 placed at position A and position B, respectively, is 4.3%. In addition, when the maximum and minimum efficiencies are measured in different positions of the target to be charged, an efficiency difference is about 7.5%. Since the efficiency is reduced due to the changes in impedance in different positions of the target to be charged on the wireless charging apparatus 100, frequency tracking for making up for the efficiency reduction is required. This frequency tracking refers to determining a frequency that minimizes the efficiency difference between different positions, by shifting the frequency to a maximum efficiency point for the changed impedance by adjusting the frequency within an inband permitted by frequency regulations.
A structure of a conventional wireless charging apparatus for determining the optimal frequency is described with reference to FIG. 3. Referring to FIG. 3, the conventional wireless charging apparatus has a Phase Locked Loop (PLL) 410 that receives a feedback output from a wireless power transmitter 400, and finds and tracks the optimal output frequency by adjusting the frequency.
In this conventional apparatus, the PLL performs frequency tracking by adjusting the frequency. However, regulations establishing allowable output power at each frequency are strict, as set forth in the frequency regulations. Specifically, when frequency tracking is performed within the inband permitted by the frequency regulations, regulations on the output power are not restrictive, whereas when frequency tracking is performed in an outband, regulations on the output power are very strict.
However, the conventional wireless charging apparatus with a PLL will monitor the output power at the shifted frequency after shifting the frequency. In other words, the conventional apparatus merely measures the power at the reference frequency. For a reference frequency of 6.78 MHz, an inband is 6.78 MHz±15 kHz. Therefore, when power exceeding the power limit before arrival at the boundary frequency (e.g., 6.765 MHz or 6.795 MHz) between the inband and the outband occurs in the outband, the conventional apparatus cannot provide advance detection.
In addition, since the PLL is a relatively expensive device, in addition to a need for a frequency tracking method capable of replacing the PLL-based method, there is also a need for a method capable of complying with the frequency regulations by detecting the output exceeding the power limit.